604 MANUAL OF PHYSIOLOGY. 



during ordinary respiration, some little change in tension of the 

 air contained in the cavity of the drum would occur and impair 

 the hearing ; and, secondly, the vibrations of the air in the pharynx, 

 produced by the voice, would enter the drum directly, and give 

 rise to an exaggerated shouting noise. 



CONDUCTION THROUGH THE LABYRINTH. 

 Every motion of the oval base of the little stirrup bone causes 

 a wave to pass along the liquid in the labyrinth. The bony case 

 of the internal ear being firm, and its contained liquid like 

 most liquids quite incompressible, the wave travels through all 

 the parts of the internal ear. Through the cochlea it can arrive 

 at the yielding membrane covering in the round opening, which 



FIG. 235. 



Diagram of the membranous labyrinth. a, 6, c, semicircular canals opening into the 

 ventricle d; e, the saccule from which the uniting canal leads into the membranous 

 canal of the cochlea, g. (Cleland.) 



separates the cavities of the tympanum and the cochlea. To 

 pass from the oval vestibular opening which is closed by the 

 stapes, to the inner tympanic membrane which closes the scala 

 tympani of the cochlea, the waves have a very complex route. 

 From the liquid lying around the membranous labyrinth the 

 perilymph the waves pass up the fluid in the vestibular spiral 

 of the cochlea, and arriving at its summit, they descend by the 

 tympanic spiral to the round opening. In this course they pass 

 at first over and then under the fluid contained in the membran- 

 ous canal of the cochlea endolymph in which the special nerve 

 terminations of the cochlea are placed. 



For the construction of the labyrinth the student is referred 



